This invention relates to ferrite articles for electromagnetic wave absorbers, which consist of at least two different ferrite layers. The laminated ferrites compared to monolithic ferrites have intentionally controllable frequency ranges which are useful as electromagnetic wave absorber.
With the advancement of communication in information-oriented modern society, various interferences, which are now termed as wave pollutions, are encountered due to unwanted electromagnetic waves. For example, the `ghost` phenomenon on television sets is caused by the complex reflected waves by tall buildings and malfunctions of electronic equipments are often caused by electromagnetic waves of outside sources. In order to prevent such interferences various modifications of transmitting and receiving method have been considered as a possible solution. (H. Yamashita, et al., "Electromagnetic wave absorbing wall for TV ghost suppression," Jap. J. Elec. Comm., J61-B(8)) However, the fundamental solution for wave interferences is to cut-off all the incoming waves from outside sources. To separate a building from unwanted wave, this would mean cladding the outside of the building with a material capable of absorbing electromagnetic waves.
One of the best known electromagnetic wave absorbers is magnetic material such as ferrites in which incoming electromagnetic waves are attenuated by transforming into heat. The magnetic loss of ferrites thus prevents electromagnetic waves from reflecting.
Basic structure of ferrite electromagnetic wave absorbers consists of a ferrite layer attached to a metal plate which is a reflecting material FIG. 1A. For such a structure the input impedance of front ferrite layer (when normalized as an impedance in free space) becomes ##EQU1## where .lambda., Z.sub.o, .mu..sub.r, .epsilon..sub.r and d represent the frequency in free space, impedance in free space, permeability, permittivity and the thickness of the ferrite, respectively. In this case, attenuation becomes as follows, EQU Attenuation (dB)=20 log(.vertline.S.vertline.) (2)
where, ##EQU2##
As shown in the above equations the characteristics of ferrite as electromagnetic wave absorbers are determined by the inherent electromagnetic properties of ferrites such as complex permeability and permittivity, which in turn are determined by the composition of ferrites. Moreover ferrite can act as the wave absorber in a narrow frequency range due to its resonance characteristics. The frequency of electromagnetic noises generated by the electronic equipments mostly ranges from 30 MHz to 1 GHz although noise frequency of each equipment is confined to a relatively narrow range. A ferrite with a composition suitable for the purpose should be developed, which is a difficult task.
As the prior methods to change the absorbing frequency range, a method wherein polymers such as a rubber were added to a ferrite has been proposed (U.S. Pat. No. 4,116,906). However, the resulting materials were only useful for noises at X-band. The materials also had various physical and mechanical problems such as degradation and low strength, which were caused by polymers added.